The construction of a transformer is prescribed by IEC (International Electrotechnical Communication) Standards Pub. 950, etc. That is, these standards provide that at least three insulating layers be formed between primary and secondary windings in a winding, in which an enamel film which covers a conductor of a winding be not authorized as an insulating layer, or that the thickness of an insulating layer be 0.4 mm or more. The standards also provide that the creeping distance between the primary and secondary windings, which varies depending on the applied voltage, be 5 mm or more, that the transformer withstand a voltage of 3,000 V applied between the primary and secondary sides for a minute or more, and the like.
According to such the standards, as a currently prevailing transformer has a structure such as the one illustrated in a cross-section of FIG. 2. In the structure, an enameled primary winding 4 is wound around a bobbin 2 on a ferrite core 1 in a manner such that insulating barriers 3 for securing the creeping distance are arranged individually on the opposite sides of the peripheral surface of the bobbin. An insulating tape 5 is wound for at least three turns on the primary winding 4, additional insulating barriers 3 for securing the creeping distance are arranged on the insulating tape, and an enameled secondary winding 6 is then wound around the insulating tape.
Recently, a transformer having a construction which includes neither the insulating barriers 3 nor the insulating tape layer 5, as shown in FIG. 1, has started to be used in place of the transformer having the construction shown in the cross-section of FIG. 2. The transformer shown in FIG. 1 has an advantage over the one having the construction shown in FIG. 2 in being able to be reduced in overall size and dispense with the winding operation for the insulating tape.
In manufacturing the transformer shown in FIG. 1, it is necessary, in consideration of the aforesaid IEC standards, that at least three insulating layers 4b (6b), 4c (6c), and 4d (6d) are formed on the outer peripheral surface on one or both of conductors 4a (6a) of the primary winding 4 and the secondary winding 6 used.
As such a winding, a winding in which an insulating tape is first wound around a conductor to form a first insulating layer thereon, and is further wound to form second and third insulating layers in succession, so as to form three insulating layers that are separable from one another, is known. Further, a winding in which a conductor enameled with polyurethane is successively extrusion-coated with a fluororesin, whereby extrusion-coating layers composed of three layers structure in all are formed for use as insulating layers, is known (JU-A-3-56112 ("JU-A" means unexamined published Japanese Utility Model application)).
In the above-mentioned case of winding an insulating tape, however, because winding the tape is an unavoidable operation, the efficiency of production is extremely low, and thus the cost of the electrical wire is conspicuously increased.
In the above-mentioned case of extrusion of a fluororesin, since the insulating layer is made of the fluororesin, there is the advantage of good heat resistance and high-frequency characteristic. On the other hand, because of the high cost of the resin and the property that when it is pulled at a high shearing speed, the external appearance is deteriorated, it is difficult to increase the production speed, and like the insulating tape, the cost of the electric wire becomes high. Further, in this case of the insulating layer, there is a problem that, since the insulating layer cannot be removed by dipping in a solder bath, the insulating layer on the terminal has to be removed using less reliable mechanical means, and further the wire must be soldered or solderless-connected, when the terminal is worked for the insulated wire to be connected, for example, to a terminal.
On the other hand, a multilayer insulated wire is put to practical use, wherein multilayer extrusion-insulating layers are formed from a mixture of a polyethylene terephthalate as a base resin with an ionomer prepared by converting part of carboxyl groups of an ethylene/methacrylic acid copolymer to metal salts, and wherein the uppermost covering layer among the insulating layers is made of a polyamide (nylon). This multilayer insulated wire is excellent in cost of electrical wire (nonexpensive materials and high producibility), solderability (to make possible direct connection between an insulated wire and a terminal), and coilability (that means that, in winding the insulated wire around a bobbin, the insulating layer is not broken to damage the electrical properties of the coil, when, for example, parts of the insulated wire are rubbed with each other or the insulated wire is rubbed with a guide nozzle)(U.S. Pat. No. 5,606,152, and JP-A-6-223634 ("JP-A" means unexamined published Japanese patent application)).
Further, to improve heat resistance, the inventors proposed an insulated wire whose base resin is changed from the above polyethylene terephthalate to polycyclohexanedimethylene terephthalate (PCT).
However, although the heat resistance of these multilayer insulated wires is acceptable to heat-resistance Class E in the test method in conformity to Annex U (Insulated wires) of Item 2.9.4.4 and Annex C (Transformers) of Item 1.5.3 of the IEC 950-standards, it is not acceptable to heat-resistance Class B of the IEC standards. It cannot be said that such multilayer extrusion-coating insulated wire satisfactorily meets the demand for improvement in the performance of transformers in the future, which will become more and more strict.
First, as electrical/electronic equipments have been made small-sized in recent years, the influence of heat generation on a transformer becomes remarkable easily, and therefore, even in the case of the above three-layer extrusion coating insulated wire, higher heat resistance is demanded. Further, the frequency used in circuits of transformers is made into high frequencies, and therefore improvements in electrical properties at high frequencies are demanded.
To solve such problems, an object of the present invention is to provide a multilayer insulated wire that is excellent in heat resistance, solderability, high-frequency characteristic, and coilability, and that is favorably suitable for industrial production.
Further, another object of the present invention is to provide a transformer excellent in electrical properties and high in reliability, wherein, when it is used at high frequencies, the electric properties are not lowered, and influence by the generation of heat is prevented, and wherein such an insulated wire excellent in heat resistance, solderability, high-frequency characteristic, and coilability is wound.
Other and further objects, features, and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings. DISCLOSURE OF INVENTION
The above objects of the present invention have been attained by the following multilayer insulated wire and the following transformer in which the said wire is used.
That is, according to the present invention there is provided:
(1) A multilayer insulated wire comprising a conductor and solderable extrusion-insulating layers made up of two or more layers for covering the conductor, wherein at least one insulating layer is formed by a mixture comprising 100 parts by weight of a resin (A), of at least one selected from the group consisting of polyetherimide resins and polyethersulfone resins, 10 parts by weight or more of a resin (B)p, of at least one selected from the group consisting of polycarbonate resins, polyarylate resins, polyester resins, and polyamide resin, and 15 to 200 parts by weight of an inorganic filler (C), PA1 (2) The multilayer insulated wire as stated in the above (1), wherein the mixture, which comprises the resins (A) and (B), and the inorganic filler (C), comprises 100 parts by weight of the resin (A), 10 to 70 parts by weight of the resin (B), and 20 to 140 parts by weight of the inorganic filler (C), PA1 (3) The multilayer insulated wire as stated in the above (1) or (2), wherein insulating layers other than the insulating layer made of the mixture that comprises the resins (A) and (B) and the inorganic filler (C), are made of a resin mixture that comprises 100 parts by weight of the resin (A) and 10 parts by weight or more of the resin (B) PA1 (4) The multilayer insulated wire as stated in the above (1) or (2), wherein insulating layers other than the insulating layer made of the mixture that comprises the resins (A) and (B) and the inorganic filler (C), are made of a resin mixture that comprises 100 parts by weight of the resin (A) and 20 to 70 parts by weight of the resin (B), PA1 (5) The multilayer insulated wire as stated in the above (1), (2), (3), or (4), wherein the insulating layer made of the mixture that comprises the resins (A) and (B) and the inorganic filler (C) is formed at least as the outermost layer, PA1 (6) The multilayer insulated wire as stated in one of the above (1) to (5), wherein the resin (A) is a polyethersulfone resin, PA1 (7) The multilayer insulated wire as stated in one of the above (1) to (5), wherein the resin (B) is a polycarbonate resin and/or a polyarylate resin, PA1 (8) The multilayer insulated wire as stated in one of the above (1) to (5), wherein the resin (A) is a polyethersulfone resin, and the resin (B) is a polycarbonate resin and/or a polyarylate resin, PA1 (9) The multilayer insulated wire as stated in one of the above (1), (2), (3), (4), (5), (6), (7), or (8), wherein the resin (A) is a polyethersulfone resin having a repeating unit represented by the following formula: ##STR1## PA1 (10) The multilayer insulated wire as stated in one of the above (1) to (9), wherein the inorganic filler (C) comprises at least one selected from among titanium oxide and silica, PA1 (11) The multilayer insulated wire as stated in one of the above (1) to (10), wherein the inorganic filler (C) has an average particle diameter of 5 .mu.m or less, PA1 (12) A multilayer insulated wire, comprising the multilayer insulated wire stated in one of the above (1) to (11) whose surface is coated with a paraffin and/or a wax; and PA1 (13) A transformer, wherein the multilayer insulated wire stated in one of the above (1) to (12) is utilized.
wherein n is a positive integer,
Herein, the outermost layer in the present invention refers to the layer situated farthest from the conductor out of the extrusion-coating insulating layers.